Abraham offers the three angels on their way to Sodom refreshment of milk and a “young calf without blemish”.... how would this have been done without compromising the kosher rules of not consuming meat and dairy at one sitting?

First of all, Non-Jews are not subject to the waiting periods between milk and meat and the angels were disguised as Arab desert dwellers. A Jew though, must wait a long time after meat before eating dairy, but does not have to wait long after dairy before eating meat. You will notice that Abraham served the dairy first.

Regards, Eliahu Levenson